1. Field of the Invention
The present invention relates to an information recording medium, a recording apparatus for recording information onto the information recording medium, and a reproduction apparatus for reproducing information from the information recording medium. More particularly, the present invention relates to a technique for recording main information together with auxiliary information.
2. Description of the Related Art
Optical discs represented by DVD (Digital Versatile/Video Disc) are widely used as media for storing a large volume of digital data, such as AV (Audio Video) data, computer data, or the like. For example, reproduction-only optical discs, in which a high-quality motion picture of two hours or more is recorded, are currently on the market. In order to protect unauthorized duplication of such copyrighted digital contents onto other recording media, a technique called “contents encryption” has been introduced. (Nikkei Electronics, Nov. 18, 1996, P. 13–14).
In this technique, compressed digital contents, such as movies, are encrypted using three-layered private keys (a title key, a disc key, and a master key) and then is stored in a user information region accessible for the user. The master key, which is the most important of the private keys, is provided only to licensed, authorized manufacturers. The disc key and the title key, which are required for each DVD and each title, respectively, are encrypted based on the master key and are stored in a control information region which cannot be accessed by the user. In this case, the user cannot access the private keys which are required for decryption, resulting in prevention of unauthorized duplication by a file copying operation or the like. Such a protection technique has the following drawback. If the contents in the entire recording region including the control information region storing the private keys are unauthorizedly duplicated onto other discs, the encrypted contents can be decrypted and reproduced by an authorized reproduction apparatus from these optical discs.
To solve the above-described problem, Japanese Laid-Open Publication No. 11-126426 discloses the following technique. Normal main information is recorded by changing the length or space between pits or recording marks formed on a track so that a resulting reproduction signal varies in a cycle which is an integral multiple of a predetermined fundamental cycle. Auxiliary information, such as an identifier for identifying an optical information recording medium, or the like, is recorded by displacing the pits or marks slightly back and forth from their normal positions.
The above-described conventional example will be described with reference to FIG. 7. FIG. 7 shows a conventional optical information recording apparatus 501. As shown in FIG. 7, the optical information recording apparatus 501 records digital information onto an optical information recording medium 502 by modulating recording light L to be applied to the optical information recording medium 502. The optical information recording apparatus 501 comprises a modulation circuit 504, a disc identifier generation circuit 505, a second modulation circuit 506, an optical modulator 507, a spindle servo 508, and a recording laser 509.
The modulation circuit 504 generates a first modulated signal S2 by switching the signal levels of first digital information D1 from a digital audio tape recorder 503 in a cycle which is an integral multiple of a fundamental cycle.
The second modulation circuit 506 generates a double modulated signal SC by modulating the timing of a change in the level of the first modulated signal S2 in accordance with a second digital information SC1 (in this conventional example, a disc identifier which is output by the disc identifier generation circuit 505) other than the first digital information D1. The optical modulator 507 modulates the recording light L in accordance with the double modulated signal SC.
FIG. 8 shows the second modulation circuit 506. FIG. 9 shows timing charts of elements in the second modulation circuit 506. Referring to FIG. 8, an EFM signal S2 which has been input to the second modulation circuit 506 is input to monostable multivibrators 510A and 510B. The monostable multivibrators 510A and 510B detect the rising edge and falling edge of the EFM signal S2 and then outputs a rising edge detection pulse MMS and a falling edge detection pulse MMR (FIG. 9). A data selector 511A receives the rising edge detection pulse MMS and a signal obtained by delaying the rising edge detection pulse MMS using a delay circuit 512A. A data selector 511B receives the falling edge detection pulse MMR and a signal obtained by delaying the falling edge detection pulse MMR using a delay circuit 512B. A disc identification information SC1 is initialized by a frame clock FCK. An M sequence code MS is generated based on a channel clock CK generated by a PLL 513. A control signal MS1 is generated by the exclusive OR operation of the initialized disc identification information SC1 and the M sequence code MS. The data selectors 511A and 511B receive the control signal MS1 and the inverted control signal MS1, respectively, via their respective selection input terminal.
When the control signal MS1 indicates “H”, the data selector 511A selects the rising edge detection pulse MMS delayed by the delay circuit 512A and outputs it as a rising edge pulse SS. When the control signal MS1 indicates “L”, the data selector 511A selects the non-delayed rising edge detection pulse MMS and outputs it as a rising edge pulse SS. When the control signal MS1 indicates “L”, the data selector 511B selects the falling edge detection pulse MMR delayed by the delay circuit 512B and outputs it as a falling edge pulse RR. When the control signal MS1 indicates “H”, the data selector 511B selects the non-delayed falling edge detection pulse MMR and outputs it as a falling edge pulse RR.
These rising edge pulse SS and the falling edge pulse RR are reconstructed into an EFM signal by a set/reset flip flop 514. As a result, the edge of the EFM signal is partially delayed. According to such a recording technique, even if the contents in the entire recording region are unauthrizedly duplicated bit by bit onto other optical discs, disc identification information which is encoded by the edge of the recording mark cannot be duplicated. Therefore, for example, when an attempt is made to reproduce information from such a duplicated disc, disc identification information cannot be detected and reproduced and the reproduction operation can be arrested.
However, such a conventional technique has the following drawback. Bit shift may occur due to dust or scars on a disc in detecting auxiliary information encoded together with main information. In this case, the edge of the EFM signal established in recording may not be correctly associated with the corresponding level of the control signal, i.e., the delay of the EFM signal may not be correctly detected. In this case, whereas the first digital information Dl can be correctly reproduced by error correction, auxiliary information cannot be correctly detected.